The present invention is generally directed toward methods of creating various laser-defined structures on pressure relief devices, namely those that comprise a rupturable member. In particular, the methods utilize a laser having a field of view that is smaller than the overall dimensions of the laser-defined structure to be milled into the pressure relief device. As a result, the structure must be milled piecemeal via a tiling process. The working surface of the pressure relief device is divided into an assortment of tiles and each tile is individually milled with the laser and/or pressure relief device being shifted in between milling of different tiles. Accordingly, methods of the present invention permit creation of generally large milled structures on the surface of pressure relief device using a laser with a relatively small field of view.
Structures for controlling various operational characteristics of rupture discs, such as opening pressure, disc reversal pressure, and disc petal geometry, have been created through various machining techniques. Traditionally, these control structures, such as lines of disc opening, have been created by contacting the surface of the disc with a die resulting in the displacement of metal in the area of contact with the die and formation of a “score” line. The die-scoring methods typically do not result in removal of metal from the disc, but rather a change in the metallic grain structure in the area of die contact. Generally, this change involves a compression and work hardening of the disc metal. This work hardening can increase the brittleness of the metal and create stress zones. The brittleness and stress zones limit the service life of the pressure relief device as a result of fatigue cracking and stress corrosion. Additionally, it is difficult to achieve a high degree of control over the depth of the score in disc manufacturing, given that the die itself becomes worn during the scoring process and must be periodically replaced. This lack of control imparts a degree of unpredictability into the disc opening characteristics.
In order to avoid the issues with die scoring, alternate processes have been devised to form lines about which the rupture disc opens. One such process is described in U.S. Pat. No. 7,600,527, which discloses forming a line of weakness through an electropolishing method. In this process, a rupture disc is provided with a layer of resist material. A laser is then used to remove a portion of the resist material corresponding to the desired line of weakness. Next, the disc undergoes an electropolishing operation to remove metal from the surface of the disc thereby forming a line of weakness having a desired depth. However, controlling the width of the electropolished line of weakness can be difficult, particularly if the disc material is relatively thick and requires extended electropolishing times in order to achieve the desired line depth. This characteristic can limit the ability to use this method in forming more complex structures comprising multiple trenches in close proximity to each other.
It has also been suggested to directly use a laser to machine a line of weakness into a rupture disc. U.S. Patent Application Publications 2010/0140264 and 2010/0224603 are exemplary in this regard. Also exemplary of the use of lasers to create various control structures in pressure relief devices are U.S. Provisional Patent Application No. 61/720,800, filed Oct. 13, 2012, and entitled “Pressure Relief Device Having Laser-Defined Line of Opening,” U.S. patent application Ser. No. 13/552,165, filed Jul. 18, 2012, entitled “Rupture Disc Having Laser-Defined Reversal Initiation and Deformation Control Features,” and U.S. patent application Ser. No. 13/780,729, filed Feb. 28, 2013, and entitled “Rupture Disc Having Laser-Defined Line of Weakness with Disc Opening and Anti-Fragmentation Control Structures,” all of which are hereby incorporated by reference in their entireties. However, heretofore, the practice of laser machining of disc control structures has been practically limited to rupture discs of a certain size based upon the field of view of the laser used in the machining process.
All scanning lasers comprise optics (apertures, lenses, etc.) that establish an operational field of view. The operational field of view is essentially the working range of the laser across any given substrate, and particularly the maximum space, as measured in degrees or units of distance, between areas that can be machined when the laser and working surface are fixed in position relative to each other. It has generally been considered undesirable to effect any relative shifting between the laser and/or rupture disc during laser milling operations. Because these laser milling operations involve removal of some, but not all, disc material on any particular location of the rupture disc, precise control over the depth of material removal is an important aspect of the milling operation. The depth of material removal is dependent generally on the total amount of energy supplied by the laser that impinges upon any particular point of the working surface. The more ablative energy that is supplied by the laser the greater the depth of the control structure that is created.
In order to avoid creation of heat affected zones within the disc material, and in order to achieve the desired depth of disc material removal, laser milling of the disc control structures often occurs as a plurality of successive passes of the laser beam across the surface of the disc. Care must be taken in plotting the laser paths followed during these passes so that the laser beam does not inadvertently impinge upon a point of the disc surface more times than is required to achieve the desired depth. If the laser beam were permitted to deliver a greater than desired amount of energy to a particular point, such as at the intersection of transverse lased trenches, the depth of material removal at the point of intersection may be greater than desired, and in some instances may penetrate through the disc material entirely. In order to avoid these concerns, relative movement between the laser and the working surface has been prohibited, as the relative repositioning of the laser and working surface increases the risk of removing a greater amount of disc material than desired, possibly leading to undesirable disc control characteristics. In a practical sense, this means that the entire control structure being milled in the disc must reside within the field of view of the scanning laser being used.
Generally, the larger the field of view of the laser, the more expensive the laser is. Accordingly, the capital costs associated with laser acquisition generally restrict laser milling of control features to small-diameter rupture discs. There are many industrial applications for large-diameter pressure relief devices, including large-diameter rupture discs and explosion vents, that would benefit from the use of control structures such as those described in the above-listed references. However, laser milling of large-scale control features has been limited because of the inability to conduct milling operations in such a pressure relief device beyond the field of view of the laser.